Numerous pharmaceutical drug release systems exist for controlled delivery of drugs in the bloodstream. These systems have been developed to maintain drug levels within a therapeutically desirable range in the bloodstream, to protect drugs that have short in vivo half-lives, and to prevent harmful side effects resulting from drug administration at high doses. These pharmaceutical drug release systems may also allow less frequent drug administration (Langer).
An ideal system for drug delivery via the bloodstream would be capable of circulating for long periods in the bloodstream, would release an entrapped drug at a controlled rate, would have the potential to reach target sites via the bloodstream, and ultimately, would be cleared or broken down without release of toxic by-products.